1. Field of the Invention
The present invention relates generally to broadcast information systems, and more specifically to a low power broadcast system for transmitting information to passerby.
2. Description of the Prior Art
As is well known in marketing and related fields, it is often desirable to communicate selected information to people traveling in automobiles. Perhaps the most ubiquitous form of communication is the roadside sign. Roadside signs have the advantage that they are, if properly designed and placed, easily read by anyone driving by.
They have several drawbacks, however, in that the amount of information which can be placed on a sign is limited, the information is not easily changed, and drivers must take their eyes off the road in order to read them. A system which communicated information to automobile drivers audibly would have several significant advantages over the use of road side signs.
The United States Federal Communications Commission (FCC) has promulgated regulations which allow the public to transmit radio signals in the AM broadcast band, which ranges from approximately 500 KHz to 1,600 KHz. Certain restrictions apply, such as limitation of broadcast power, and transmitting antenna length. The antenna is limited to 3 meters in length, which is very short for the wavelengths involved. Also, the total DC power supplied to the output stage transistor cannot exceed 100 milliwatts. Within the restrictions of the regulations, anyone is free to broadcast within this band.
In order to take advantage of these FCC regulations, several manufacturers have built low power transmitters in compliance therewith. These transmitters incorporate an endless loop cassette tape which contains the message to be broadcast. The endless loop tapes typically have a playback length of one to two minutes. The tapes are played continuously, so that the broadcast message is repeated with a period equal to the playback time of the endless loop tape.
Such prior art systems have a number of serious drawbacks which have precluded their widespread acceptance. Such systems are subject to mechanical breakdown of the cassette tape drive system, which usually operates continuously. The tape itself usually has to be replaced at least monthly. In addition, the range of prior art systems is quite short, thereby limiting their usefulness. An additional factor precluding acceptance has been the difficulty of using endless loop tapes for continuously repeating messages. Each tape has a fixed playback length, which requires that messages be exactly that length. Shorter messages leave a period of dead time in transmission until the message repeats. If, for example, a message is 70 seconds in length, and only 60 and 90 second endless loop tapes are available, the message must either be shortened to fit on the 60 second tape, or 20 seconds of dead air time will occur between each repetition of the messages when the 90 second tape is used. As is well known in the broadcasting industry, dead air time is extremely undesirable.
It would be desirable for a low power broadcast information system to provide increased reliability and transmitting range. It would further be desirable for such a system to be easy to use, and to allow the repetitive broadcast of messages of any length without dead air time between repetitions of the message.